Plumb Redistribute "allow duplicates" property to Dataflow

[celeste-zeng]

The DataflowGroupByKey is identical to the Beam GroupByKey transform except for having a allowDuplicates property plumbed from the Redistribute transform.

Added corresponding override for RedistributeByKey, a translator for DataflowGroupByKey, and override builder in the runner.

Tested internally by running a load test and passed all Github actions tests.

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[celeste-zeng]

R:@kennknowles

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[kennknowles]

This looks good. Let's add the other test suites, and also do internal testing.

[kennknowles]

Very nice! This became a very clear and minimal result that gets the job done well. (yes, there is a lot of boilerplate, but that is Java and Beam for you)